This invention relates to a method for establishing a telecommunications link by means of bursts in a digital radio network having at least one other digital radio network superposed thereon, and to a telecommunications unit for establishing a telecommunications link by means of bursts in a digital radio network having at least one other digital radio network superposed thereon.
A prior-art digital radio network is described in a book by M. Mouly and M.-B. Pautet entitled "The GSM System for Mobile Communications", published by the authors in 1992, Palaiseau, France. The digital radio network described in that book is the "Global System for Mobile Communciations" (GSM), whose technical components meet the GSM standard.
In that book it is described on pages 232-237 and particularly in subchapters 4.3.1.1 and 4.3.1.3 that in GSM, training sequences are used for the transmission of radio signals, e.g. for synchronization. A single training sequence is used within a so-called synchronization burst, and eight different training sequences are used within so-called normal bursts, with the indices of the eight training sequences of the normal bursts being transmitted in the synchronization burst in coded form. By means of their training sequences, interfering similar GSM bursts which reach a receiver nearly simultaneously can be distinguished. Training sequences are inserted as specific bit sequences in the middle of the bit structure of a burst in order to minimize the distance to the information bits located at the beginning and end of the bit structure.
In the area of two superposed digital radio networks there is the risk of both digital radio networks using identical training sequences. Then, mobile stations, for example, can exchange bursts with both digital radio networks and decode such bursts, which results in mutual interference. This is possible during the synchronization phase of mobile stations, but also in their idle state or during the time of a call on an assigned traffic channel. Such interference is conceivable particularly between a public land mobile network (PLMN) and a cordless telephone system (CTS), since especially in a CTS, frequency planning for avoiding interference cannot be carried out.